The contractile mechanism of vertebrate striated muscle is activated by the binding of calcium to troponin. Studies relating force to free calcium concentration show that the calcium activating mechanism has the properties of a cooperative system. One aspect of this cooperativity is that calcium-receptor interactions can be modulated by cross-bridge attachments formed between myosin and actin filaments. The aim of this study will be to analyze and interpret the calcium-troponin interaction as a dynamic property of the working myofilament system. A double isotope technique and EGTA buffers will be used to make simultaneous measurements of bound Ca 2 ion, force development, and ATPase activity in rabbit psoas muscle fibers extracted with detergent and glycerol. Questions to be answered are: 1) what is the quantitative relationship between receptor occupancy, ATPase activity, and the mechanical output?, 2) how do mechanical parameters such as force and length alter calcium receptor properties?, and 3) in the intact myofilament system what is the influence on calcium binding of variations in chemical environment which are known to alter mechanical activity? These studies should enhance our understanding of how calcium mobilization and binding are controlled by the mechanical and chemical conditions which prevail during the contraction cycle of living muscle.